Method for electrolinetic downhole logging

ABSTRACT

A method and apparatus for measuring the properties, such as permeability, of the rock surrounding a metal cased borehole by generating a seismic or sonic shock downhole within the borehole which is propagated into the surrounding formation where it generates an electrokinetic signal which is detected by measuring the potential between at least two space apart electrodes in contact with the casing.

The present invention relates to a method and equipment for obtaininginformation concerning the rock and soil surrounding a borehole.

The measurement of permeability of rocks surrounding a borehole isimportant in assessing the location of water or oil reserves, includingthe quality and quantity of the reservoir rock. Existing methods areunable to measure the permeability of a porous rock directly with anyaccuracy from a downhole tool.

In addition to its value in the assessment of the quality and quantityof water or oil reservoirs, rock permeability is very important indetermining at what rate and at what cost these fluids can be producedfrom boreholes.

Downhole logging is known in which equipment is lowered downhole, aseismic or sonic signal is generated by a seismic or sonic shock and issent out from the borehole, this seismic signal generates an electricfield and an electromagnetic signal is received and can then be analysedto obtain information concerning the rock or soil surrounding theborehole. The generation of an electric signal by this means is calledan electrokinetic signal (EKS).

U.S. Pat. No. 3,599,085 describes a method in which a sonic source islowered down a borehole and used to emit low frequency sound waves.Electrokinetic effects in the surrounding fluid-bearing rock cause anoscillating electric field in this and is measured at least twolocations close to the source by contact pads touching the boreholewall. The electromagnetic skin depth is calculated from the ratio ofelectrical potentials and the permeability of the rock deduced. U.S.Pat. No. 4,427,944 and the equivalent European Patent 0043769 describe amethod which injects fluid at high pressure from a downhole tool togenerate electrokinetic potentials; these are measured by contactelectrodes against the borehole wall. The risetime of the electricalresponse is measured and from this the permeability of the porous rockis determined.

Patent Application PCT/GB96/02542 discloses a method of measuring theproperties of rock surrounding a borehole in which a seismic pulse isgenerated downhole which propagates outwards from the borehole toproduce electrokinetic signals which are detected within the boreholeand used to measure the properties of the surrounding rock.

In these methods the borehole is not cased i.e. there is no metal casingsurrounding the borehole, the seismic or sonic signal is propagateddirectly into the surrounding formation and the electrokinetic signalgenerated is received from the formation. If the borehole were cased themetal casing would prevent the reception of electromagnetic signalswithin the borehole as the casing would act as an electromagnetic shieldor cage; this means that these methods can only be used before the wellis cased.

In use, operational wells are cased and so the prior art methods cannotbe used for downhole logging in operational wells and this is alimitation on the application of EKS downhole logging. It EKS downholelogging could be used in cased wells this would greatly expand theapplication of such methods and would enable there to be regularmonitoring of the rock structure surrounding a hole or a number of holeswhilst in production. Such information can be generated as required,with minimal interference of drilling operations and can facilitate theselection of the location of step out wells in a structure and detectdiscontinuities and other information concerning the rock formation. Inaddition, after a production well is no longer in production, EKSdownhole logging could be used to monitor the surrounding rock and togenerate further knowledge about the structure and its features withoutrequiring the drilling of extra holes.

We have surprisingly found that it is possible to produce and detectelectrokinetic signals returned from the surrounding rock formation fromthe metal casing in a cased well.

When a well is cased and surrounded by a metal casing, usually made ofsteel, the returned electromagnetic signals, which are weak would beexpected to be shorted out by the casing and so no signal would bereceived between electrodes in contact with the casing inside theborehole.

We have surprisingly found that it is possible to detect such signalsand we have now devised a method for EKS logging in a cased well.

According to the invention there is provided a method for measuring theproperties of an earth formation traversed by a borehole which is casedby a metallic casing in which a seismic or sonic shock is generateddownhole within the borehole and is propagated into the surroundingformation and an electrokinetic signal generated by the seismic or sonicshock is detected by at least two spaced apart electrodes in contactwith the casing.

The invention also comprises apparatus for detecting electrokineticsignals generated by a seismic or sonic shock generated downhole in aborehole in which apparatus there are at least two spaced apartelectrodes adapted to make contact with the well casing in the casedborehole and a means connected to the electrodes which is able, inconjunction with the electrodes, to detect the electrokinetic signals.

It has been surprisingly found that, by contacting the casing with atleast two spaced apart conductors, an electrical signal can be detectedwhich has been generated by the seismic shock. Although the casing actsas a low-value resistor in parallel with the conductors it has beenfound, in practice, that not all the signal is shorted out.

The electrodes which are in contact with the casing are spaced apart sothat the signal is generated between them and fed to the amplifier. Theconductors can be in the form of a dipole and suitable separation of theconductors is from 0.05 m to 2 m.

The conductors preferably make good contact with the casing and can bein the form of spring loaded brushes as in conventional contacts, orrolling wheels which can cut through debris to make contact. Conductorscan make contact in a localised area or can be in the form of a ringwhich fits inside the borehole and makes contact along itscircumference.

Alternatively a plurality of pairs of electrodes can be positionedcircumferentially around the borehole.

In one embodiment of the invention the electrical receiver preferablyconsists of at least one pair of electrodes forming a short dipoleantenna with electrically isolated ends. For each pair the ends areconnected to an amplifier which amplifies the signals whilst keepingthem electrically isolated; this is carried out by referring thepotential of each end independently to a floating reference potential.The signals are preferably amplified and converted to digital formbefore being communicated (e.g. by wire) to the surface for recordingand processing.

The amplifier chosen is one which can be used with very low impedancesources. For the preferred results the amplifier can deliver anamplified signal at the frequency of the received signal, and theamplifier preferably has sufficient open loop gain at this frequency togive a detectable and measurable signal. The frequency is preferably inthe range 1 Hz to 100 KHz and a gain of at least 25 decibels ispreferred.

An amplifier which can be used which includes operational amplifiers ofthe OP37 type which can deliver a signal from a 0.1 ohm signal which canbe used in the present invention. Use of and the AD849 type is alsopossible.

The means for generating the seismic signals preferably generates aseries of pressure pulses or, more preferably, a continuous pressureoscillation, at one or more finite frequencies. It may consist of amechanical vibrational device, an electromagnetic device, a sparkersource, an explosive source, an airgun operated hydraulically orelectrically or any other such conventional sonic source designed foruse on a downhole tool but preferably it should be a magnetostrictive orpiezoelectric transducer whose signal is controllable electrically. Theterm “seismic pulse” can include a pulse which can be referred to as asonic or acoustic pulse.

A preferred means for enabling the seismic signal to be generatedradially comprises a cylindrical chamber having holes in its side, whichwhen downhole will be fall of drilling fluid with the sides of thechamber being close to the sides of the borehole, there being a means totransmit a shock or applied force to the fluid in the chamber so as tocause the shock to be transmitted through the fluid in the chamberthrough the holes into the surrounding rock. The holes should bedistributed substantially uniformly around the casing so that the shockis transmitted in all directions. The shock or force can be applied byany of the means referred to above.

It has been found that a seismic or sonic shock can propagate throughfluid in the borehole and through the casing into the traversedformation to generate electrokinetic signals.

The seismic signal can be generated whilst the apparatus is lowered orraised up from the borehole, thus providing a continuous orsemi-continuous measurement of rock along the borehole.

When there is a means lowered into the borehole to generate a seismic orsonic shock pairs of electrodes can be positioned above and below themeans for generating the seismic or sonic shock.

In one embodiment, in which the borehole is cased as the borehole isdrilled, the sonic shock can be generated by a drill bit drilling theborehole and the detection of the electrokinetic signals by the methodof the invention can be carried out whilst the borehole is drilled.

In the equipment of the present invention it is convenient to use one ortwo electrical receivers placed above and below the acoustic source, thecase of the dipole antennae preferably aligned vertically orhorizontally above and below the seismic source.

Preferably the means for detecting the electrical signals compares thepotential at the ends of the electrodes. The potential at the ends ofdipole antenna are compared by connecting them to an amplifier in whichthe potentials are preferably referred to a non-earthed reference (avirtual earth) and these new potentials are amplified and compared. Sucha procedure allows amplification with very little distortion of thepotential to be measured and with a high degree of common-mode noiserejection and is superior to other conventional methods ofamplification. Preferably the non-earthed reference potential is that ofa common line in the amplification and data acquisition circuitry of thereceiver and is not connected directly to earth.

Preferably there is provision for isolating and balancing the signalsfrom each of the electrodes before they reach the amplifier circuit inorder to giver the maximum common-mode rejection of electromagneticnoise. This balancing can be achieved manually before running in a givenborehole to compensate for variations in electrode performance in agiven hole or by means of a suitable electronic circuit givingcontinuous feedback whereby continual adjustment can be made.

The seismic source preferably continuously emits sound simultaneously onat least two finite frequencies with the resultant oscillation the sumof the various sinusoidal pressure oscillations. Preferably if twofrequencies are used these frequencies are between 1 Hz and 100 KHz e.g.about 1 KHz and 5 KHz.

Preferably the amplified electrical signals arc demodulated with respectto the source frequencies and the amplitude and phase relative to thesource sampled at a frequency of about 1-100 Hz per channel andconverted from analogue to digital form, of 16 bit accuracy or more. Thedigital data transmitted to surface is recorded as a data file and canthen be processed.

The amplitude and response of the electrokinetic response to an acousticpulse have been shown to be closely related to the electrokineticcoefficient and the permeability of the target porous rock respectively.For a sonic oscillation of a known frequency the amplitude and phase ofthe electrical response with response to the source is a function ofboth electrokinetic coefficient and permeability; however, measurementof amplitude of response on two frequencies allows each of theseproperties of the rock to be determined independently. After processinga log of rock permeability, electrokinetic coefficient, electricalconductivity and porosity van be produced. Alternatively, if theamplitude and phase of the electrokinetic response at a single frequencyare measured are measured, the permeability and porosity may be derivedfrom these.

It is believed that the method of the present invention makes use of anelectrokinetic effect in which the seismic wave generated by the seismicsource and, passing through the interface of the borehole casing withthe surrounding porous rock and through interfaces within the rock wherethe fluid properties change, stimulates electrical signals detected atthe receiving electrodes or coils. The seismic oscillations within theporous rock give rise to fluid flow within the rock and as cations andanions adhere with differing strengths to capillary walls, a resultingelectric dipole is generated within the rock. This electric dipoledistorts the quasi-static electric field within the slightly conductingmedium of the rock and this distortion propagates back to the tool,where it is measured.

The invention is described with reference to the accompanying drawingsin which:

FIG. 1 illustrates the invention diagrammatically and

FIGS. 2 and 3 show the characteristics of amplifiers suitable for use inthe invention.

Referring to FIG. 1 there is a borehole (1) cased by a steel casing (2)and filled with liquid (mud etc. in the case of a well being drilled orproduced liquid on the case of a production well). The downhole tool (4)is connected to a cable (5) so that it can be raised and lowered downborehole (1). In the tool are two electrodes (5) and (6) in contact withthe casing. There is a seismic source (4) which consists of a hammer andpiston and a cylindrical chamber with holes disposed uniformly about it.

In use, the tool (4) is lowered down the borehole (1) until it is inposition. The hammer and piston are activated to produce a seismicsignal comprising continuous acoustic oscillations by compression of thefluid which fills the borehole and the seismic signal propagates throughthe fluid and the casing into the surrounding formation (7).

The seismic signal generated is shown by the solid arrows and generatesan electrokinetic signal at A, this electrokinetic signal is detected bythe electrodes (5) and (6) and the amplitude and response time of thesignal measured using an amplifier and passed to converter whichconverts then from analogue to digital form. These signals are thenpassed via data connectors to a computer which controls, samples andrecords the data and finally processes and displays them.

Referring to FIGS. 2 and 3, FIG. 2 is a plot of the gain in dB againstfeed back resistance in ohms for an OP37 amplifier and FIG. 3 is a plotof the open loop gain at 5 KHz against the signal attenuation in dB.This shows that for high feedback resistance the amplifier can deliversignificant gain from a low impedance (0.1 ohm) and so can be used toamplify the signals received through the well casing.

What is claimed is:
 1. A method for measuring at least one property ofan earth formation traversed by a borehole having a casing comprising:(a) placing at least two spaced apart electrodes in contact with saidcasing; (b) generating a sonic shock within said borehole which ispropagated into the earth formation; (c) generating electrokineticsignals by said sonic shock; and (d) detecting a signal from saidelectrodes as a measure of at least one property of the earth formation.2. The method of claim 1 including: (a) forming said electrodes as adipole antenna; and (b) amplifying the signal from said electrodes. 3.The method of claim 2 in which said signal is generated with a frequencyin the range of 1 Hz to 100 kHz and producing a gain of least 25decibels by the amplifier.
 4. The method of claim 1 including the stepof lowering or raising said electrodes while the sonic shock isgenerated and while said eletrokinetic signals are received.
 5. Themethod of claim 1 including the step of drilling said borehole toproduce said sonic shock and detecting said electrokinetic signals whilesaid drilling continues.
 6. The method of claim 1 including the step ofcomparing the potentials of the electrodes and referring them to anon-earthed reference (a virtual earth) and thereafter amplifying theresultant signals.
 7. The method of claim 1 wherein said at least oneproperty is the permeability of said earth formation.
 8. Apparatus fordetermining at least one property of an earth formation transversed by aborehole comprising: (a) a casing in the borehole; (b) means forgenerating a sonic shock propagating into the formation and generatingelectrokinetic signals; and (c) at least two electrodes in electricalcontact with said casing for generating an electrical signal indicativeof at least one property of the formation.
 9. The apparatus of claim 8wherein said casing is metallic.
 10. Apparatus as claimed in claim 8 inwhich the ends of the electrodes are connected to an amplifier in whichthe potentials are referred to a non-earthed reference (a virtual earth)and these new potentials are amplified and compared.
 11. Apparatus asclaimed in claim 8 including means for isolating and balancing thesignals from each of the electrodes before they reach the amplifiercircuit.
 12. Apparatus as claimed in claim 8 in which said electrodesare spring loaded brushes.
 13. Apparatus as claimed in claim 8 in whichsaid electrodes include rolling wheel means for cutting through debristo make contact with said casing.
 14. Apparatus as claimed in claim 8 inwhich said electrodes include a ring which fits inside the borehole andmakes contact along its circumference.